docs: def z_axis_rotation(theta):
          """Generates a 3x3 rotation matrix for a rotation of angle
          theta about the z axis.
  
          Parameters
          ----------
          theta : float
              amount to rotate, in radians
  
          Returns
          -------
          :obj:`numpy.ndarray` of float
              A random 3x3 rotation matrix.
          """
          R = np.array([[np.cos(theta), -np.sin(theta), 0],
                        [np.sin(theta), np.cos(theta), 0],
                        [0, 0, 1]])
          return R